The invention relates generally to optical fibers and more specifically to optical fibers with a controlled refractive index profile.
Beam splitters and combiners are widely used today in optical communications systems. Optical splitters divide a single light beam into a plurality of light beams. Conversely, combiners take a plurality of light beams and combine them into a fewer number of beams (e.g. eight to one). Combiners and splitters generally introduce two losses into the overall system performance, xe2x80x9cinsertion lossxe2x80x9d and xe2x80x9cexcess loss.xe2x80x9d Insertion loss is the loss in a transmission line that results from inserting (i.e., introducing) the component in the transmission line (e.g., a 3 dB 2xc3x972 coupler inserted into a line will introduce a 50% or 3 dB loss in the line). Excess loss is the internal loss attributable to the specific component itself.
Excess loss arises, for example, from fiber fusion and/or tapering of the fiber, and is generally related to the number of splits or combinations of fibers made in the component. (Fusion refers to localized heating and softening of two or more fibers to attach them together; tapering relates to reduction in the size of the fiber geometry, e.g., as a result of fusion or polishing.) For example, an 8-to-1 splitter will generally have a higher excess loss than a 4-to-1 splitter designed for the same operation band, with the same fibers. Fibers can be joined by means of cladding-fused and cladding-polished couplers.
With reference to FIG. 1, a typical single mode optical fiber 100 used to create either a cladding-polished or cladding-fused coupler includes a core 110 and a cladding 120. In a cladding-fused coupler, shown in FIG. 2, the claddings of two single mode fibers 100a, 100b are fused together, thereby fixing cores 110a, 110b in close proximity to each other at the region of joinder. Cross-talk between cores 110a, 100b results in transfer of a portion of the signal propagating through one of the cores to the other core. In cladding-polished couplers, the core 110 of each fiber is polished flat and mechanically aligned to the other core 110, thereby inducing cross-talk between the cores.
The coupling efficiency of both cladding-fused and cladding-polished couplers is reduced because a portion of the signal propagating in the core is transferred to cladding 120 at the point of coupling, thereby increasing the excess loss of the device incorporating the coupled fibers.
Accordingly, there exists a need for an optical fiber with reduced or minimal excess loss resulting from coupling.
The present invention is directed to improving the coupling efficiency of both cladding-fused and cladding-polished optical couplers thereby reducing excess loss.
One aspect of the invention is directed to an optical fiber including a core having a diameter dc and a refractive index nc, a primary cladding concentrically surrounding the core and having an outer diameter dpc, and a refractive index npc that is less than the core refractive index nc and a secondary cladding concentrically surrounding the primary cladding. The secondary cladding has an outer diameter dsc and a refractive index nsc that is less than the primary cladding refractive index npc. The primary cladding outer diameter dpc is a fraction of the secondary cladding outer diameter dsc. The difference between the primary cladding refractive index npc and the secondary cladding refractive index nsc is greater than or equal to a constant and less than or equal to the core refractive index nc less the constant.
In other embodiments, the primary cladding is updoped or the secondary cladding is downdoped. The primary cladding outer diameter is a fraction of the secondary cladding outer diameter. The fraction may range from  greater than 0% to 75%; is desirably 10% to 50%; and is most preferably around 20%. The above-mentioned constant relating the refractive indices of the primary and secondary claddings is preferably equal to or greater than 0.0002.
In another embodiment, the cladding and/or the core includes a material inducing photosensitivity, such that exposure to radiation creates a refractive-index modulation within the cladding and/or the core.
In another aspect, the invention is directed to a method of coupling a pair of optical fibers. The method includes the steps of providing a pair of fibers and fusing the fibers along a portion of their longitudinal extents to couple them.
The primary claddings may or may not be fused. Thus, in some embodiments, the primary claddings are fused, while in other embodiments, the primary claddings are not fused but the secondary claddings are fused proximate to the primary claddings.